1. Field of the Invention
The present invention relates to a method of fabricating a high quality ZnO thin film, which is a new II-IV material that can replace ultraviolet (UV) and blue light emitting diodes (LED""s), laser diode (LD) emission devices based on GaN semiconductors that are III-V nitrides. The ZnO thin film of the present invention can also replace UV detectors utilizing AIN-GaN alloy (AlxGa1-xN)m, is devoid of any green-yellow deep-level emission peak typically observable at room temperature in bulk crystals, and emits only a pure near band edge (xe2x80x9cNBExe2x80x9d) UV corresponding to an energy level of 3.3 eV. The present invention also relates to a method of fabricating UV emission and detection devices utilizing such high quality ZnO thin film as their base material, and to a method of fabricating thin films that can replace III-V nitrides for use in UV/blue LED""s and LD""s.
2. Description of the Prior Art
ZnO thin films have been used as the base material for reducing gas sensors such as SnO2 and as photoconductive material. Because it is easy to grow crystals of ZnO along the c-axis, it has been used as thin film material for varisters and surface acoustic filters utilizing its good piezoelectric characteristics. On the other hand, ZnO of the II-VI group has a direct bandgap structure with an optical energy bandgap of 3.37 eV at room temperature.
Up to now, SiC based compounds of the IV group, ZnSe based compounds of the II-VI group, and GaN of the III-V group compound has been used as the thin film material for UV emission devices.
SiC has an advantage that addition of p-type or n-type impurities and control of conductivity are easy. Thus, SiC has been developed in the late 1980""s as material for millicandela (mcd) level low-brightness blue LED""s. However, SiC is not suitable for high-brightness blue LED""s because SiC has an indirect energy bandgap structure.
Therefore, ZnSe based material having a direct energy bandgap has been used for high brightness blue LED""s. The characteristics of ZnSe crystal is good, because ZnSe has a low lattice mismatch with GaAs used as the substrate. However, because molecular beam epitaxy (MBE) is typically used to grow ZnSe crystals, it is difficult to add and control elements such as Zn, Se, Mg, or S having high element pressure and to add p-type impurities.
GaN of the III-V group compound, which was developed in the 1990""s, has a direct bandgap structure and a very high lattice mismatch with sapphire substrates. Thus, the dislocation density between the sapphire substrate and the GaN thin film is great. Nevertheless, GaN shows very good characteristics of UV/blue emission. However, because such great dislocation density adversely affects the lifetime of high-power laser diodes, it was necessary to develop a new buffer material.
In this regard, ZnO has been proposed as the buffer material between sapphire substrates and GaN, because ZnO has the same Wurtzite crystal structure as that of GaN and the lattice mismatch between ZnO and GaN is only 2.2%. By using ZnO as the buffer material between the sapphire substrate and GaN, it is possible to reduce the dislocation density in the boundary between the sapphire substrate and GaN caused by the great lattice mismatch (16.7%) when growing the GaN thin film.
ZnO is a wide bandgap (3.37 eV) semiconductor having a direct energy bandgap. Thus, the optical characteristics of ZnO are similar to those of GaN used as material for conventional UV/blue emission diodes (LED""s) and LD""s. Especially, ZnO has a high excitation binding energy (60 meV) at room temperature, resulting in more efficient emission than GaN. Also, ZnO has a low threshold energy for stimulated spontaneous emission by laser pumping. In addition, it is possible to grow thin films of ZnO at a lower temperature (500-600xc2x0 C.) than that required for III-V nitrides, because III-V nitrides require preheating of the substrate at a temperature in excess of 1000xc2x0 C. Therefore, there are various options for the material used as the substrate when growing a ZnO thin film.
However, conventional methods of fabricating ZnO thin films had a disadvantage that the ZnO thin film was Zn-rich due to oxygen deficiency, such that the ZnO thin film is grown as a nonstoichiometric thin film having the characteristics of an n-type semiconductor. Other disadvantages were that, because of the effect of other impurities, the ZnO thin film showed a greenish-yellow peak due to deep-level defect having an energy of about 2.4-2.6 eV along with a UV peak due to NBE shift having an energy of about 3.37 eV, when it was subject to an optical characteristics test by using photoluminescence (PL). Such deep level defect peak due to impurities adversely affects the efficiency and characteristics of high-purity UV/blue emission devices and thus should be eliminated.
Conventional methods of fabricating ZnO thin films include reactive e-beam evaporation, sputtering, chemical vapor deposition (CVD), and spray pyrolysis. However, these conventional methods all had disadvantages that they failed to fabricate stoichiometric ZnO thin films and merely produced polycrystalline thin films of ZnO.
ZnO thin films to be used as optical material require high quality crystal property and uniformity. In order to fabricate such high quality ZnO thin films, methods such as metalorganic CVD, molecular beam epitaxy, and pulsed laser deposition have been recently used. However, all of these methods have a disadvantage that they are very expensive. In contrast, sputtering is a type of physical vapor deposition (PVD) and has advantages of high-speed growth of the thin film, capability of large area growth, and low cost. However, no effort has been made to grow high quality ZnO thin films by using sputtering for optoelectronic devices such as LED and LD.
Therefore, it is an object of the present invention to provide an economical method of fabricating high quality ZnO thin films showing only NBE without any deep level emission at room temperature, in order to replace GaN which is a III-V compound. It is also an object of the present invention to provide an apparatus for implementing such method of the present invention.
To this end, the method of the present invention for fabricating a ZnO thin film for use in ultraviolet detection and emission devices operable at room temperature comprises the steps of introducing argon (Ar) and oxygen (O2) into a vacuum chamber while maintaining a vacuum level of 1-100 mTorr in the vacuum chamber, preheating a substrate, depositing a ZnO monocrystal thin film on the substrate by RF magnetron sputtering while introducing carbon (C) or nitrogen (N) atoms from an atomic radical source installed over the substrate, and slowly cooling the substrate while maintaining a partial pressure of oxygen in the vacuum chamber at a partial pressure level used while depositing the ZnO thin film.
Preferably, the ratio of argon (Ar) to oxygen (O2) is less than 4/1, and more preferably in the range of 1/1 to 3/1. The substrate is preferably preheated to a temperature range of 500-650xc2x0 C. The energy density per unit effective area applied to the target is 3.9-7.4 W/cm2 during the RF magnetron sputtering. The substrate may be one selected from the group consisting of a Al2O3 monocrystal substrate, a monocrystal substrate such as Si having a large lattice mismatch with ZnO, and a substrate using the ZnO as a buffer.
In addition, the present invention provides a RF magnetron sputtering apparatus for fabricating ZnO thin films used in ultraviolet detection and emission devices operable at room temperature, wherein a target and a substrate are horizontally arranged and an atomic radical source is installed over the substrate for p-type doping.